The invention relates to a device for amplifying electronic signals, comprising:
an amplifier having an input and an output, and
a feedback loop arranged between the output and the input of the amplifier.
Such devices are commonly used in the electronics industry to amplify, inter alia, audio signals, video signals or data signals originating from read heads of hard disks. The theory regarding this type of devices is described in chapter 8 of the book entitled xe2x80x9cAnalysis and Design of Analog Integrated Circuitsxe2x80x9d by Messrs. Gray and Meyer. This book describes the advantages of the feedback loop, which enables, in particular, to stabilize the gain of the amplification device, adapt the input and output impedance of the device, linearize the amplification performed by the device, and increase the amplification bandwidth.
For some applications, it is desirable to enable the user of the amplification device to control the parameters that define the amplification bandwidth, and in particular to make the low cut-off frequency of the amplification device adjustable.
In principle, such a control requires a modification of the arrangement of the amplifier itself, or a modification of the arrangement of the feedback loop. However, these two options have a harmful effect on the proper functioning of the device. The elements forming the amplifier are in principle dimensioned so as to produce as little noise as possible during operation. A modification of the elements of which the amplifier is composed would cause the amplifier to produce more noise, which is to be proscribed. In addition, the elements forming the amplifier and the feedback loop, in principle, are dimensioned in such a way that the input impedance of the amplification device is adapted to the impedance of an element, which is arranged upstream of the device and is intended to supply the signal to be amplified to the amplification device. This applies, in particular, when the signals to be amplified originate from a read head of a hard disk via a flexible lead, in which case the transfer rate of data, and hence the frequency of the signals to be amplified, are very high. A modification of the elements forming the amplifier itself or the feedback loop would cause the impedance adaptation to be disturbed and hence result in a reduction of the high cut-off frequency of the amplification device, which is also to be proscribed.
It is an object of the invention to overcome these drawbacks by providing an amplification device wherein the bandwidth of the amplification can be easily adjusted without the performance of the device being adversely effected in terms of noise and high cut-off frequency.
In accordance with the invention, an amplification device in accordance with the opening paragraph comprises a plurality of feedback loops, which are placed between the output and the input of the amplifier, which feedback loops are each arranged so that each feedback loop has an adjustable gain and all the feedback loops jointly form an assembly having an equivalent impedance, which is substantially independent of the gain settings selected.
In this device, it is possible to modify the arrangement of the feedback loops without causing a change in the equivalent impedance of the amplification device, viewed from the input of the device. It is thus possible to modify the amplification bandwidth, more particularly the low cut-off frequency of said band, without disturbing the impedance adaptation effected by the feedback loops assembly. In addition, it is no longer necessary to modify the elements of which the amplifier itself is composed, so that the performances of the amplifier, for which the amplifier has been optimized, are preserved.
In a variant of the invention, an amplification device as described hereinabove comprises a decoupling capacitor, which is placed upstream of the amplifier, and at least one first and one second feedback loop which are placed, respectively, between the output of the amplifier and first and second terminals of said decoupling capacitor.
In this variant of the invention, the decoupling capacitor serves to suppress the DC component of the signal to be amplified and enables to dissociate the first and second feedback loops, thereby facilitating a modification of their arrangement so that the value of the low cut-off frequency of the amplification device can be modified without a modification of the equivalent input impedance of the device.
In accordance with an advantageous embodiment of the invention, each feedback loop is composed of at least one first and one second switchable impedance, a selection of the first or the second impedance being effected by means of a first and a second switch, which are to be driven by, respectively, a first and a second control signal.
This embodiment enables a simple and flexible adjustment of the low cut-off frequency of the amplification device by programming the control signals. The number of possible values of the cut-off frequency is larger as the number of feedback loops and the number of switchable impedances forming said loops is larger.
In a particular embodiment of the invention, the impedance which is equivalent to a parallel array of the first impedances of the feedback loops is substantially equal to the impedance which is equivalent to a parallel array of the second impedances of said loops.
In accordance with a preferred embodiment of the invention, the switchable impedances are resistances.
Although the invention can be employed in every type of application where an amplification of an electronic signal is required, it can particularly advantageously be used in applications where data signals are to be amplified, which data signals originate from read heads of hard disks. The invention also relates to a read system including a read head, which is used to scan an information storage medium and generate electronic signals that are representative of said information, said system further including an amplification device as described hereinabove, for amplifying said signals.